The production of microcapsules by spray-drying techniques (i.e., atomization) is known in the art. Thus, it has been long known that solutions of polymeric alcohol xanthates, such as cellulose, starch, amylose, dextran, sugar, polyvinyl alcohol, polyallyl alcohol, etc., can be converted into regenerated polymeric alcohols in the form of small, hollow spheres by spray drying at a temperature sufficient to effect a decomposition of the xanthate. The solutions which are spray dried may be the ordinary caustic-containing solutions. The presence of caustic tends to accelerate the decomposition of the xanthate during the spray drying, but has the disadvantage of tending to depolymerize the polymeric alcohol. If the solution of polymeric alcohol xanthate is decausticized, as by dialysis, dilute acid or weak acid neutralization, cation exchange, anion exchange, etc., to a pH of less than about 13 prior to spray drying, the hollow spherical particles of regenerated polymeric alcohols which are produced are contaminated with less by-product materials and do not contain excess alkali, which tends to degrade the polymers. The xanthate solutions, either in the caustic or decausticized form, may be spray dried to produce hollow spheres of the xanthate. The hollow spheres of the xanthate or the regenerated alcohol are washed, preferably with acid, to remove alkali and by-products.
More recently, other methods of producing void-containing microcapsules by atomizaton have been proposed. One such method is disclosed in copending application Ser. No. 346,391, filed Mar. 30, 1973 and now abandoned, in the names of Simon Babil and James A. Claar. As disclosed in the copending application, a polymer solution consisting of an organic polymer, a volatile solvent for the polymer and a lower volatility non-solvent for the polymer which is miscible with the solvent or a polymer emulsion consisting of an organic polymer, a volatile solvent for the polymer and a lower volatility non-solvent which is immiscible with the solvent are atomized at a sufficient temperature and pressure to volatilize off the solvent to produce multicellular microcapsules having encapsulated therein the non-solvent, following which the non-solvent is removed by evaporation to produce void-containing multicellular microcapsules.
While the above-mentioned prior art methods of preparing void-containing microcapsules have certain advantages, they possess a number of serious disadvantages which materially limit their commercial acceptability. Thus, for example, in the early prior art as exemplified by the use of polymeric alcohol xanthate solutions, a considerable number of process steps were involved, such as decausticizing, washing the microcapsules to remove alkali and by-products, and collection and subsequent utilization of the microcapsules which often involves considerable handling. Even in the more recent prior art method as exemplified by the aforementioned copending application, certain disadvantages remain. Thus, for example, in the method of the copending application, it is necessary to select polymer solvents having sufficient volatility to be volatilized at the operating conditions of the atomizer apparatus. Further, the apparatus employed in the method is somewhat extensive and includes the necessity for a microcapsule collection chamber and, finally, the problems of handling the microcapsules for subsequent utilization remain.